The present invention pertains to plasma processing of workpieces, and in particular pertains to a method and apparatus for improving the uniformity of plasma processing.
Ionized gas or “plasma” may be used during processing and fabrication of semiconductor devices, flat panel displays and other products requiring etching or deposition of materials. Plasma may be used to etch or remove material from semiconductor integrated circuit wafers, or to sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in manufacturing or fabrication processes typically is done by introducing a low-pressure process gas into a chamber surrounding a workpiece such as an integrated circuit (IC) wafer. The atoms or molecules of the low-pressure gas in the chamber are ionized to form plasma by a radio frequency energy (power) source after the gas molecules enter the chamber. The plasma then flows over and interacts with the workpiece. The chamber is used to maintain the low pressures required for plasma formation, to provide a clean environment for processing the semiconductor devices and to serve as a structure for supporting one or more radio frequency energy sources.
Plasma may be created from a low-pressure process gas by inducing an electron flow that ionizes individual gas atoms or molecules by transfer of kinetic energy through individual electron-gas molecule collisions. Typically, electrons are accelerated in an electric field such as one produced by radio frequency (RF) energy. This RF energy may be low frequency (below 550 KHz), high frequency (e.g., 13.56 MHz), or microwave frequency (e.g., 2.45 GHz).
The two main types of etching in semiconductor processing are plasma etching and reactive ion etching (RIE). A plasma etching system typically includes a radio frequency energy source and a pair of electrodes. A plasma is generated between the electrodes, and the workpiece (i.e., substrate or wafer) to be processed is arranged parallel to one of the electrodes. The chemical species in the plasma are determined by the source gas(es) used and the desired process to be carried out.
A problem that has plagued prior art plasma reactor systems is the control of the plasma to obtain uniform etching and deposition. In plasma reactors, the degree of etch or deposition uniformity is determined by the design of the overall system, and in particular by the design of the RF feed transmission and the associated control circuitry.
In a plasma reactor system, the electrode is connected to a RF power supply. The technological trend in plasma reactor design is to increase the fundamental RF driving frequency of the RF power supply from the traditional value of 13.56 MHz to 60 MHz or higher. Doing so improves process performance, but increases the complexity of reactor design.
One approach to improving etch and deposition uniformity has been to use a multi-segment electrode. With reference to FIG. 1, plasma reactor system 8 comprises reactor chamber 10 having an interior 12, within which is arranged a segmented electrode 16 having separate thick conducting electrode segments 18 each with an upper surface 18U and a lower surface 18L. A silicon slab or “facing” (not shown) may be attached to each of the lower surfaces 18L of the segmented electrode by suitable attachment means to control contamination due to sputtering of the metal electrode. Electrode segments 18 are separated by an insulator 20 and are powered by corresponding RF power supplies 26 via RF feed lines 30 connected to respective electrode segments. Power control to electrode segments 18 is provided by a main control unit 36. Match networks 40 arranged between RF power supplies 26 and electrode segments 18 are tuned to provide the best match to the load associated with a plasma 50 formed in interior region 12, so as to optimize power transfer to the plasma.
Reactor system 8 includes a workpiece support member 60 opposite segmented electrode 16, upon which a workpiece 66, such as a wafer, is supported. The design of segmented electrode 16 is such that lower surfaces 18L of electrode segments 18 interfaces with a vacuum region 70 in interior region 12. This puts electrode segments 18 directly in contact with plasma 50 formed in vacuum region 70, although if silicon facings as mentioned above (not shown) are used, the surfaces of the silicon electrode facings will be directly in contact with plasma 50. Numerous seals (not shown) are required between insulators 20 and the electrode segments, and between the chamber 10 and insulators 20, to isolate vacuum region 70.
Current plasma reactor systems can perform an etch process with approximately 5% non-uniformity. This level of performance is sufficient to meet near-term needs for state-of-the-art process performance, but will soon be inadequate as the demands on the manufacturing process increase to require, on a routine basis, non-uniformity below 5%.
In light of the demands on improving process speed, one technological trend in plasma reactor design is to increase the fundamental RF frequency from the traditional value of 13.454 MHz to 60 MHz or higher, as mentioned above. Doing so improves process performance, but increases the complexity of reactor design. A second trend in reactor design is to have multiple electrodes, i.e., electrode segments, such as those discussed above in connection with FIG. 1. However, multiple electrodes combined with increased operating frequencies mean that delivering the correct amount of RF power becomes more complicated because of capacitive coupling between the electrode segments and greater sensitivity to parasitic capacitive and inductive elements. This effect is exacerbated by the shorter wavelengths of higher fundamental frequencies. The result is increased difficulty in reducing process non-uniformity.
In addition, current multi-segmented electrode plasma reactors require a power supply for each electrode. Thus, if there are five electrode segments, there must be five corresponding power supplies (or separate amplifiers). This leads to high cost and increased maintenance requirements and thus high wafer processing costs. This cost and increased maintenance might be worthwhile if there were a way to improve the performance of such a system to provide a higher degree of etch or deposition uniformity beyond the present limits of existing plasma processing systems.